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A193109
Least k such that 2^x + k produces primes for x=1..n and composite for x=n+1.
1
0, 1, 9, 3, 225, 15, 65835, 1605, 19425, 2397347205, 153535525935
OFFSET
1,3
COMMENTS
All terms except the first four are congruent to 15 mod 30.
a(10) was found in 2005 by T. D. Noe and a(11) was found in the same year by Don Reble.
Other known values: a(13) = 29503289812425.
a(12) > 10^13. - Tyler Busby, Feb 19 2023
MATHEMATICA
Table[k = 0; While[i = 1; While[i <= n && PrimeQ[2^i + k], i++]; i <= n || PrimeQ[2^i + k], k++]; k, {n, 9}] (* T. D. Noe, Jul 21 2011 *)
PROG
(PARI) is(k, n) = for(x=1, n, if(!isprime(k+2^x), return(0))); 1;
a(n) = {my(s=2); forprime(p=3, n, if(znorder(Mod(2, p))==(p-1), s*=p)); forstep(k=s*(n>1)/2, oo, s, if(is(k, n) && !isprime(k+2^(n+1)), return(k))); } \\ Jinyuan Wang, Jul 30 2020
CROSSREFS
Another version of A110096.
Sequence in context: A302536 A038293 A119516 * A242069 A197828 A116393
KEYWORD
nonn,more
AUTHOR
STATUS
approved